1. Field of Invention:
This invention relates generally to medical instruments for testing by means of cutaneous sensation a patient's threshold of pain, and more particularly to an electronic algesimeter which includes a pointed probe that is pressable against the skin of the patient at any desired site, the algesimeter accurately indicating and holding the value of pressure imposed on the skin at the pain threshold to provide a threshold reading whereby by testing the patient from time to time and recording the readings, a diagnostician can from this history determine the changing condition of the patient.
2. Status of the Art
The skin consists of two superposed layers, the epidermis and the dermis. Associated with the skin are receptor organs of touch, temperature, pain and other cutaneous sensations. These receptors include Meissner's corpuscle and Merkel's disc which when stimulated elicit the sensation of touch. The Pacinian corpuscle responds to deep pressure, while stimulation of free nerve endings gives rise to the sensation of pain.
Cutaneous sensations are transmitted from the skin to various regions in the cerebral cortex. Thus, sensations of touch as well as those of warmth and coolness are perceived and interpreted in particular regions of the somesthetic or bodily sensory area of the parietal lobe. The sensation of pain for each side of the body is conducted to the brain independently. Hence, the sensitivity of a particular area of the body may be compared with a reference area on the opposite side thereof. This comparison is useful in diagnosis, for the involvement of the sensory nervous system is unilateral.
Certain neurological and other medical examinations and procedures entail the location and delineation of areas of analgesia or hyposthesia, the former being the absence of pain or touch sensitivity; the latter, reduced pain or touch sensitivity. Also of interest is hyperalgesia, that is, enhanced sensitivity to painful stimuli. Such tests are normally carried out in special examinations for the treatment of neurological disorders as well as in general medical check-ups and in diagnosing various diseases.
Soma-sensory dysfunction due to lesions in the sensory nerves or tracks is at least as common as motor disturbances in clinical neurology. When an injury or disease adversely affects a peripheral nerve, per se, or at different levels of its course to the brain, it is manifested as motor or sensory defects, or as a combination thereof. When there is an involvement of the sensory system, it may affect all sensory modalities or the impairment may be restricted to a particular form of sensation. Two patterns of preferential sensory loss have been recognized. In one, selective loss of pain and temperature develops, while in the other, there is a loss of touch and pressure sensitivity. A third pattern is the simultaneous involvement of all modalities of sensations.
Conventional skin sensitivity test procedures, though useful in screening examinations, are neither quantitative nor selective. The usual clinical procedure to test the sensory function employs a sharpened pin to prick the skin area, the patient reporting to the examiner whether or not he can feel the stimulus and, if he can, the intensity. The examining physician will then ask the patient to compare sensations experienced in the same region on two sides of the body. Abnormality is recognized by the difference in the elicited stimuli. This method is altogether unsatisfactory, save when the defect is very gross, for mild to moderate defects, though significant, are generally missed.
The main drawback incident to the conventional test procedure is that the physical characteristics of the stimuli are essentially undefined variables and the thresholds are not specific, nor are they quantified or validated. The results of such tests are therefore quite indefinite and are not reproducible. The physician is forced to rely on the patient's memory and ability to compare the previous stimuli to the present one. Thus neither the patient nor the doctor can determine whether the applied pin prick stimuli are equal in quantitative rather than in subjective terms. The physician himself can be mistaken because his pin prick stimuli may vary in intensity from site to site and from time to time.
These deficiencies become more pronounced when between each testing there is a substantial gap of a few hours, or of several days or months. In such cases, the physician must depend entirely on the patient's ability to comprehend and remember the differences from previous test occasions. Moreover, the physician himself is not sure whether the pin prick stimuli given on these occasions were of identical magnitude. And if the patient has to be examined at various times by different doctors, the test results become even more unpredictable, for there is no standardization of the applied sensory stimuli.
The need to keep a record of the stimuli required to produce a painful sensation is vital in cases involving nerve injury, surgical repair of the nerves, tropical diseases such as leprosy, and common stroke. In these cases, the recovery usually takes a protracted course with surgical or medical treatment. Hence a periodic review of the sensory function of the affected area is an invaluable guide in arriving at a correct prognosis, and it assists the doctor in deciding whether to interfere medically or surgically at any particular stage during the course of the disease.
As pointed out in the Kerokian U.S. Pat. No. 3,074,795, with a conventional sensitivity test procedure, the muscular coordination required of the medical examiner to make pin pricks with the proper degree of uniformity is exceedingly difficult to attain, particular in older practitioners. In many situations, therefore, the difference in the pain sensitivity reported by the patient may arise, not because of his medical condition, but by reason of variations in the intensity of the pin strikes made by the medical examiner. Thus, in some instances, variations in pain felt by the patient may not indicate the actual pain sensitivity of the skin but may simply be due to the uncertain muscular coordination of the examiner. The factors outlined above render the testing of pain sensation by conventional techniques unreliable, and the results thereof statistically inaccurate.
In order to provide a highly compact skin sensitivity detector, the patent to Fisher, U.S. Pat. No. 2,704,539, discloses a pen-like instrument having a needle secured to one end of a compressible spring disposed within a tubular casing, the needle going through a bore in the front end of the casing. The other end of the spring is attached to a plunger within whose axial bore is a rotatable shaft having a spiral groove, an indicator pin carried by the plunger extending into this groove. The Fisher arrangement is such that when the needle is pressed against the skin of a patient at a selected site, this acts to axially shift the plunger and thereby rotate the shaft and to cause the indicator pin to advance along a pressure-indicating scale.
The advantage of the Fisher arrangement is that it indicates the magnitude of the applied pressure at the threshold of pain. However, the Fisher device has many practical drawbacks. Thus, in order to obtain a threshold reading, the patient must tell the examiner when he experiences pain at the instant it is felt. The examiner must then look at the instrument to read the indicated pressure. Inevitably, there will be a time delay in the reaction time of the doctor to the patient's verbal response to the sensation of pain.
And since the threshold indication on the Fisher device reflects the prevailing degree of pressure exerted on the skin of the patient by the doctor holding the instrument, the doctor may find it difficult to exactly maintain this pressure in the period when he is taking a reading. Consequently, in the time elapsing between the patient's verbal response to pain and the moment the doctor takes a reading, the exerted pressure may change and an inexact reading may be obtained. Further, since in Fisher the needle passes through a bore and encounters friction as it is axially shifted by pressure, this, too, impairs the accuracy of the pressure reading. Another disadvantage of Fisher's meter is that it is non-linear, for the helical spring therein does not have a linear characteristic.
Also of general background interest are the patents to Kanatani, U.S. Pat. No. 4,313,446; Karatsu, U.S. Pat. No. 1,259,820 and Gluzek, U.S. Pat. No. 2,453,841.